In a traditional dump truck, a vehicle body and a body (also referred to as a hoist or vessel) are connected via a hydraulic hoist cylinder. The body is lifted up and down by extending and retracting the hoist cylinder. The hoist cylinder is extendable by changing a flow of hydraulic oil supplied from a hoist pump using a hoist valve (Patent Document 1).
When a large amount of hydraulic oil is supplied to the bottom of the hoist cylinder in order to move the body upwardly, an amount of oil supply considerably exceeds an amount of oil returned to a hydraulic oil tank. Accordingly, an oil level is suddenly lowered in the hydraulic oil tank, so that inner pressure within the hydraulic oil tank is considerably lowered. Thus, suction pressure within the hoist pump is also considerably lowered, so that cavitation may occur in the hoist pump to damage the hoist pump. Especially at high altitude, the pressure within the hydraulic oil tank is remarkably lowered since atmospheric pressure is lowered.
Thus, to prevent cavitation, in some instances, the inside of the hydraulic oil tank may be pressurized in advance by a breather, or air may be delivered into the hydraulic oil tank from an air source such as a compressor to pressurize the inside of the hydraulic oil tank in advance so as to supplement the predicted decrease in the inner pressure within the hydraulic oil tank.    Patent Document 1: JP-A-7-52701